Erosthenes.cs

void Erosthenes(int n)
{
  bool[] primes = new bool[n + 1];
  for (int i = 0; i < primes.Length; i++)
  {
    primes[i] = true;
  }

  for (int i = 2; i < Math.Sqrt(n) + 1; i++)
  {
    if (primes[i - 1])
    {
      for (int j = (int)Math.Pow(i, 2); j <= n; j += i)
      {
        primes[j - 1] = false;
      }
    }
  }

  int count = 0;
  for (int i = 2; i < primes.Length; i++)
  {
    if (primes[i - 1])
    {
      Console.WriteLine(i);
      count++;
    }
  }
  Console.WriteLine($"There are {count} primes up to {n}");
}

Erosthenes(100);